1. Field of the Invention
The present invention relates to an optical information read method for reading information from an optical recording medium (hereinafter referred to as the optical disc) by driving and controlling a modulated optical waveform. The present invention also relates to an optical disc drive that implements the above optical information read method.
2. Description of the Related Art
The storage capacities of optical disc drives have been enlarged so that CDs using a conventional infrared laser (having a wavelength of 780 nm) and DVDs using a red laser (having a wavelength of 650 nm) are widely used. Further, Blu-ray Discs (BDs) using a blue laser (having a wavelength of 405 nm) are also commercialized and recently marketed.
When reading the information recorded on an optical disc, it is necessary to minimize semiconductor laser noise. The semiconductor laser noise is called “RIN” (Relative Intensity Noise) and generally expressed by the following equation:RIN=10×log(ΔP2/P2×1/Δf)(dB/Hz)where ΔP is an AC component of optical output, P is a DC component of optical output, and Δf is a measurement bandwidth. The definition formulated in a written BD standard states that the RIN must be not higher than −125 dB/Hz.
A high-frequency superposition method (HF: High Frequency) is known as an RIN reduction method for use with an optical disc read device. This method turns on and off a laser oscillation at a high frequency to maintain a multi-longitudinal-mode oscillation state, thereby suppressing changes in the optical output of a laser diode that are caused by reflected-light feedback from an optical disc. The RIN is related to the HF modulation factor of a semiconductor laser (the ratio between PHF and Pa (PHF/Pa) where PHF is a light pulse peak power prevailing at the time of high-frequency superposition and Pa is the average power of all high frequencies), and controlled by adjusting the HF modulation factor.
Meanwhile, the oscillation characteristics of a semiconductor laser vary with temperature. Therefore, when the operating temperature varies, the power of emitted light also varies. An invention disclosed in Japanese Patent Application Laid-Open Publication No. 2006-48885 provides a drive device for a semiconductor laser (laser driver (LDD)) with a temperature sensor, superposes a sub-current pulse, which compensates for a temperature-induced change, over a main current pulse, which drives the semiconductor laser, when the two pulses are in the same phase, and supplies the result of superposition to the semiconductor laser to avoid a temperature-induced recording pulse power change.
Another invention disclosed in Japanese Patent Application Laid-Open Publication No. 1993-299738 provides an optical disc drive that drives a semiconductor laser by a high-frequency superposition method. This optical disc drive observes the light emission intensity of the semiconductor laser at appropriate timing when a high frequency current is superposed over a drive current supplied from a laser driver to the semiconductor laser and when such high frequency is not superposed the drive current, and individually controls the amplitude of a reference current (DC current) for driving the laser and the amplitude of the high-frequency current to be superposed, thereby maintaining the modulation factor of the laser within a definite range.